Simultaneous measurements of the transmembrane action potential (TAP) and isometric contraction were made in right ventricular papillary muscles isolated from senescent and young adult rat hearts. In muscles contracting at the peak of the length-tension curve, contractile tension developed in response to excitation (DT), and the maximum rate of tension development (dT/dt), were not age-related; contraction duration (CD) was 17 percent greater in senescent than in young adult (p less than .001); resting membrane potential (RMP) was not age-related. TAP time above zero (T pos) and integrated area above zero (A pos), and times to 75 percent (T75) and 90 percent (T90) repolarization were approximately twofold greater in senescent than in young adult (p less than .001). An increase in [Ca++]e to 2.5 mM: increased DT, dT/dt, CD, T pos, and A pos in both age groups; decreased T75 in both groups; but increased T90 in sensecent only. The transient change in A pos with each beat following the step increase in [Ca++]e was highly correlated with the changes in DT and dT/dt. The changes in steady state T75 and T90 due to the change in [Ca++]e was significantly correlated with those in CD, DT and dT/dt in senescent but not in young adult. We conclude that TAP and contractile parameters in rat myocardium demonstrate interrelationships similar to those observed in other species; both TAP and CD are prolonged in senescent versus young adult; a prolonged and greater extent of depolarization appears to be related not only to the prolonged CD in senescent but may also have a role in determining the peak force developed in response to excitation.